1. Field of the Invention
This invention relates to a circuit for converting pressure variation to a variation in the frequency of a periodic electrical output signal of the circuit.
2. Prior Art
Many capacitive transducers have been designed to provide a variation in electrical capacitance as a function of some other parameter. The other parameters may include pressure acting upon the "plates" of the capacitive transducer, the aligned areas of such plates, the positions of the plates as a function of mechanical movement, the type of dielectric material between the capacitive plates, and combinations of these. All of these capacitive transducers, however, have a common characteristic; they have a variation of an electrical capacitance that can be detected in a way that can provide a desired indication of the magnitude of the capacitance or its change and of the cause of such capacitance magnitude or change where the cause is the desired information.
Many prior art techniques are known for detecting capacitance variations. The electrical capacitance may be used, for example, to control an oscillator's frequency output and any capacitance variation can be made to cause a related variation in the output frequency. In U.S. Pat. No. 3,886,473 to Heyck, a capacitive transducer is described that converts a small displacement (such as could be caused by the effect of a pressure change on an aneroid) of a mechanical input member into a capacitance change. The Heyck system uses two oscillators, a reference frequency oscillator and a variable frequency oscillator which generates an electrical signal having a frequency controlled by the displacement (pressure variable) transducer. The Heyck system uses the fixed or reference frequency oscillator and the variable frequency oscillator to fill separate digital counters. The first counter, when filled, causes a gate to supply pulses to a third counter until the variable frequency oscillator has filled the second counter. Thus, the third counter contains a digital count proportional to the frequency difference between the periodic output signals from the two oscillators. The oscillators disclosed by Heyck are precision radio-frequency oscillators having both inductive and capacitive elements providing an output frequency that is inversely proportional to the square root of the product of the inductance and capacitance.
The Heyck patent mentions another system for detecting frequency variation caused by capacitance variation. This is accomplished with a fixed or reference frequency, a variable frequency due to capacitance change, a frequency equal to the difference between the fixed and variable frequencies and by use of a precision oscillator and divider to generate a precise counting period over which the difference frequency cycles are counted.
While capacitance variation can be detected as described above, the electrical circuits and necessary apparatus are very expensive and sensitive, require precise and individual calibration, and are very difficult to fabricate in large quantities in a way that would permit combined capacitive transducers and detection circuits of all to have identical transfer functions.
For mass production use of capacitive transducers and detection circuits, it is desirable to have a simple calibration method and identical transfer functions for all of the mass-produced units. If, for example, the frequency of the periodic output signal is inversely related to the capacitance that varies with a sensed parameter, it is desirable to have identical terms and coefficients (e.g., slope and offset) in the transfer function applicable to all of the mass-produced capacitive transducers and associated detection circuits. Accordingly, a simple, reliable and repeatable method of fabrication and, in particular, of calibration is very desirable.